A main component of a computer is an assembly that is referred to as a magnetic hard disk drive, HDD. The HDD includes rotating magnetic disks, write and read heads that are suspended by a suspension arm adjacent to the surfaces of the rotating magnetic disks and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disks. The read and write heads are directly located on a slider that has an air bearing surface (ABS). The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk. When the slider rides on the air bearing, the write and read heads are employed for writing magnetic impressions to and reading magnetic impressions from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
The write head includes a coil layer embedded in insulation layers (insulation stack), the insulation stack being sandwiched between first and second pole piece layers. A gap is formed between the first and second pole piece layers by a gap layer at the air bearing surface (ABS) of the write head and the pole piece layers are connected at the back gap. Current conducted to the coil layer induces a magnetic flux in the pole pieces which causes a magnetic field to fringe out at a write gap at the ABS for the purpose of writing the aforementioned magnetic impressions in tracks on the moving media, such as in circular tracks on the aforementioned rotating disk.
In recent read head designs a spin valve sensor, also referred to as a giant magnetoresistive (GMR) sensor, has been employed for sensing magnetic fields from the rotating magnetic disk. The sensor includes a nonmagnetic conductive layer, hereinafter referred to as a spacer layer, sandwiched between first and second ferromagnetic layers, hereinafter referred to as a pinned layer and a free layer. First and second leads are connected to the spin valve sensor for conducting a sense current. The magnetization of the pinned layer is pinned perpendicular to the air bearing surface (ABS) and the magnetic moment of the free layer is located parallel to the ABS, but free to rotate in response to external magnetic fields. The magnetization of the pinned layer is typically pinned by exchange coupling with an antiferromagnetic layer.
The thickness of the spacer layer is chosen to be less than the mean free path of conduction electrons through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfaces of the spacer layer with each of the pinned and free layers. When the magnetizations of the pinned and free layers are parallel with respect to one another, scattering is minimal and when the magnetizations of the pinned and free layer are antiparallel, scattering is maximized. Changes in scattering alter the resistance of the spin valve sensor in proportion to cosθ, where θ is the angle between the magnetizations of the pinned and free layers. In the read mode the resistance of the spin valve sensor changes proportionally to the magnitudes of the magnetic fields from the rotating disk. When a sense current is conducted through the spin valve sensor, resistance changes cause potential changes that are detected and processed as playback signals.
A spin valve sensor is characterized by a magnetoresistive (MR) coefficient that is substantially higher than the MR coefficient of an anisotropic magnetoresistive (AMR) sensor. For this reason a spin valve sensor is sometimes referred to as a giant magnetoresistive (GMR) sensor. When a spin valve sensor employs a single pinned layer it is referred to as a simple spin valve. When a spin valve employs an antiparallel (AP) pinned layer it is referred to as an AP pinned spin valve. An AP spin valve includes first and second magnetic layers separated by a thin non-magnetic coupling layer such as Ru. The thickness of the spacer layer is chosen so as to antiparallel couple the magnetizations of the ferromagnetic layers of the pinned layer. A spin valve is also known as a top or bottom spin valve depending upon whether the pinning layer is at the top (formed after the free layer) or at the bottom (before the free layer).
A critical performance parameter for a read head or a write head is the magnetic track width that can be read or written by the head. A write head having the smallest possible magnetic write width (MWW) while having sufficient performance is of course desired. Similarly, magnetic read sensors are designed to have the smallest possible magnetic read width (MRW) while having sufficient signal amplitude to allow the largest possible density of data tracks to be recorded on a given magnetic disk.
Since the track width, (MRW, or MWW) is such a critical parameter, one can appreciate the importance of having an accurate means of measuring the MRW for a read sensor.
Magnetic read width, MRW, is defined as the half maximum width of reader response function in the cross track direction. The definition of reader response function should be:R(x)=Lim(micro-track width-0)Amplitude of output signal(x)/width of micro-track.Where x is the position of reader in the cross track direction and micro-track is a data track on the media with a width much narrower than MRW. Therefore, a current method, that has been used to measure a read head MRW and has currently been considered the best one, has required the creation of a micro track of data. A micro track is constructed by a multi-step process that involves first recording a magnetic track, also known as a full-track, and then erasing, or “shaving off” one or both sides of the data track, leaving just a very small portion of the track that is much narrower than the actual track width of the read head as well as the write head.
This micro-track is then read while moving the read sensor from side to side until the signal drops to ½ of that which it was at its peak value. The distance between those points is defined as the value of the MRW in this micro-track measurement.
The key to having accurate and reliable results using this method is to obtain a micro-track with fixed and narrow-enough track width. However, when track pitch becomes very narrow it is very difficult to obtain such a micro-track with strong enough signal relative to be background noise level and consistent micro-track width from run to run. This is exacerbated by several factors such as TMR (Track Misregistration), spindle runout, write width variation, etc, especially at high frequencies. There also remains a great deal of disagreement over what method should best be used to generate such a micro-track. For example, some researchers believe that the micro-track should be generated using a DC erase, while others prefer an AC erase. Whether the erasure should be made on one side or both sides of the full-track is still a controversial subject now.
Therefore, as track width becomes smaller, there is a strong felt need for a means for measuring the magnetic read width (MRW) of a read head directly from a full track profile, without the need for creating a micro-track. Such a means for measuring MRW would not only eliminate a lot of unnecessary confusion and arguments regarding how to generate a micro-track, but would also allow us to have much more accurate results for the measurement of very narrow track widths, such as those of current and future disk drive products. Such a means for measuring track width would also preferably be useful in a production disk drive, since the creation of a micro-track would not be necessary.